Extreme home improvement: DIY photovoltaic array
[Note: Hover your cursor over the images for the metatext.]
Last week’s post on photovoltaic systems presented the finances without much engineering. We had a guest post this week on more photovoltaic “rent vs own” decisions, but this post will focus on the construction. When I describe the system that we built, our choices were mostly driven by finances. Back in 2004, the goal was boosting our financial independence by spending over $16K to eliminate a $100/month electric bill. When we recouped almost 60% of that expense as state/federal tax credits, the payback became even more compelling.
Photovoltaic hardware prices have dropped by nearly an order of magnitude over the last decade. These days you can hire a solar company to achieve your energy independence at the same expense as we did in 2004: no DIY labor required. If you’re in the military then you’ll want to wait until you own your home for at least the length of the payback, but even if you’re living in base housing you may have solar. If you’re stationed on Oahu then you’ll almost certainly have sunshine powering your base home.
WARNING: A photovoltaic array operates at up to 600 volts DC, 240 volts AC, and several thousand milliamps of current. Exposed wiring can kill you before you realize you’ve made a mistake. Unless you have personal experience with electrical work, please hire an electrician.
DISCLAIMER: PV systems have to comply with municipal code and permit requirements. You’ll want to hire an electrician to help with buying the right parts and signing the right paperwork.
Another disclaimer: In most installations, you’re going to be drilling holes into (what used to be) a perfectly good roof.
So why in the world would you care about this post? (If you’re a solar engineering geek like me then read on!) If you’re a normal person with more conventional interests then you’ll learn more about why the installer wants to build your photovoltaic array a certain way. By the end of this post you’ll either feel more confident about DIY, or else you’ll know it’s not for you. I’ve included two more links at the end of this post on how to shop for solar panels and how to choose an installation contractor.
For the rest of this post, I’m going to assume that you’ve chosen the simplest and cheapest configuration: grid-tied photovoltaic panels that will supply your home’s energy first and then dump their excess power into your local electric utility grid. Your system also includes an electric inverter that converts the electricity produced by a panel (a couple dozen volts of direct current or DC power) into the electricity used by your local electric utility (240 volts of alternating current or AC power).
Your panels will either have microinverters (one inverter per panel) or a string inverter (one inverter for a dozen panels). Both types of inverters have their relative advantages and are extremely reliable, but you only care about the different inverters if you’re planning to expand your system for future needs.
I’m not going to clog this post with dozens of photos because it’ll really slow down the page load times. If you’re building your own system and you’re curious about a configuration, try Google Images first and then contact me for more details. I’m happy to show you what we did, but your local code may require a different configuration
Our First 1100 Watts of Photovoltaic Panels
We bought our first 20 photovoltaic panels in late 2004. (My spouse actually spotted a “classified ad” in our local “newspaper”.) The seller had owned the panels for nearly 10 years. Each panel is only rated at 55 watts (a fraction of today’s panel power), and they were mounted on heavy-duty stainless-steel angle-iron racks. He was desperate to sell and we were willing to unbolt them from his roof. We paid $2.25/watt at a time when most panels were selling for $6-$8/watt from the factory (and over $10/watt installed).
When we saw the ad, Hawaii Electric Company had just started a residential electric net-metering program. We knew that we’d need a construction permit and an electric contractor’s signature (with his license number) to obtain HECO’s approval of our net-metering application. We were about the 25th net-metering permit on the island, and today Oahu has thousands of homes with PV arrays.
We found our electrician at a small solar installation company. He was willing to check the seller’s panels with us before we bought them (for a consultant’s fee) and he agreed to show us what parts met the code to re-install them on our roof. We knew we were getting a great price on the panels, so we didn’t mind spending a few hundred extra dollars for his consulting fee and a rented truck (a U-Haul “panel” truck!) to haul the panels home.
The solar contractor gave us a parts list (with specifications) and we started our scavenger hunt. We found lag bolts & roof sealant in home improvement stores, and we found the wiring & connectors at an electric supply store. Our south roof is a cathedral ceiling so we were able to see the beams at the eaves and follow them up to the ridge. A careful layout with chalk lines (and banging on the shingles with a rubber mallet) helped reassure us that we kept the racks centered and parallel on the beams. Even so, I still managed to drill one hole down the side of a beam (I swear it’s warped!) so the roof sealant was essential to repairs as well as construction. We added small angle brackets to the roof bolts so that we could raise & lower the PV panel racks by pivoting them on the lower angle brackets. This made it a lot easier to wire the panels together after the racks were mounted on the roof.
Wiring was a challenge. We planned to mount our inverter on the side of the house by the electric meter. (Microinverters weren’t available in 2004!) That’s 75 feet away from the panels and it goes through our garage attic. The code requires that the wires be protected from weather (and critters) by running them through PVC conduit (UV-resistant gray instead of white).
Because we planned to have the inverter control two separate strings of panels, we routed four separate wires (plus a spare wire and a ground wire) through the conduit. (Luckily we’ve never needed the spare.) We mounted a small junction box on the roof (to connect the panel wires to the conduit wires) and bolted the conduit to the roof with metal straps (hurricane defense). There’s no easy way to snake wires through 10-foot lengths of conduit inside an attic that’s only four feet high, but my labor is a lot cheaper than the electricians. They would have wanted to route conduit all the way across the roof (instead of through the attic) which would have just given a hurricane more exposed PVC to work on.
We leisurely mounted the racks and wiring for an hour or two during the cool mornings. (Financial independence made this project more enjoyable than if we’d had to take leave from work and hustle it 16 hours/day.) The electric contractor visited us a couple of times to make sure that we were on track. (He was charging by the hour for his education as well as his inspections.) By early 2005 we had the panels mounted & wired together, the wiring strung through the conduit, and we’d fixed our (minor) mistakes.
The electrician hung the inverter and connected its cables to the house’s electric distribution panel. He made it look easy but his code knowledge and experience was essential. Today I can replicate his layout (like a trained monkey) but I don’t know all the code options and I wouldn’t have been able to solve the design challenges that he met.
After the inverter was connected to the service panel, the electrician finished wiring the panels in series (up to 600 volts DC) and connected the panel string at the junction box. After a multimeter check confirmed that the panels were making voltage, he connected the last two wires to the inverter.
[Pro tip: if you must wire a photovoltaic panel, do the work away from sunlight or cover the panel with cardboard and duct tape. Electricians do this safely (“survivor bias”?) but it’s beyond the skill level of the average homeowner. One panel might not hurt you too badly if it turns sunshine into DC voltage, but a string of panels will endanger your life.]
The electrician flipped the switch, the inverter booted up, the sun came out from behind the clouds, our electric meter slowed down and… started spinning backwards! We were making more power than we were using, and our grid-tied system was dumping the excess power into HECO’s grid.
After a few engineering high-fives, the electrician shut off the inverter again. We paid him and he went home.
Why did he shut off the system? Because HECO still had to do their own inspection and sign our net-metering agreement.
“Guerrilla solar” is technically illegal, and I wrestled with my ethics. I’ve been there before: nuclear engineers are not supposed to make our own rules, but submariners have to use their own judgment & initiative. What if the electrician made a mistake? What if the design wasn’t safe after all? What if HECO wanted something changed or re-worked?
A five-minute phone chat with the electrician reinforced my confirmation bias. He said that HECO wouldn’t inspect the entire array– they’d only check the connection to the service panel. Everything else was up to the electrician, and he’d used a standard design that was already operating in hundreds of Mainland homes.
We turned the inverter back on. I’m glad we did because it took HECO two weeks to get around to their inspection. First they actually sent a fraud/theft inspector because our electrical use had dropped so precipitously that they thought we’d sneaked an extension cord over to the neighbors or tapped into a street lamp. The electric inspector had never seen a residential grid-tied photovoltaic array before. HECO finally approved the net-metering agreement several months later.
Speaking of inspections & permits, we paid the electrician to handle the construction permit. He actually subcontracted a “runner” to stand in line at our city Department of Permits & Planning. He had already filed a standard set of pre-approved plans with the DPP staff, but it still took the runner all day to finish the paperwork. We paid a $411 permit fee (on top of the $150 runner fee). The electrician charged nearly $900 to tutor us through the mechanical & electrical code, to install the inverter, and to make the connections. He also signed the net-metering agreement (with his license number) for HECO’s approval.
Flush with success, we took our maximum state/federal tax credits in 2005.
Our Second 1960 Watts
In late 2005 we found four decade-old 50-watt panels on Craigslist at $4/watt. Even better, we discovered that Evergreen Solar was selling blemished panels through eBay. The panels had broken junction wires and discolored solar cells but they had been over-engineered to still deliver rated voltage & power. I put an eBay “Buy it now” $7000 PayPal transaction (*gulp*) on a credit card to buy 16 110-watt panels. (Today’s panels are nearly three times more powerful.) It even included shipping & handling from Florida to Oahu. At less than $4/watt, the panels compared favorably to the local market’s $6-$8/watt. (Today they’d cost about 75 cents/watt.) We made more rails and brackets using scrap aluminum and mounted the panels on our roof. Following the procedure that we learned from the electrician, we wired them in series and connected them at the junction box as a second string for a total of just over three kilowatts.
DIY Solar Water Heating
In early 2006 we also added a solar water heating system. We started with two 4’x8′ panels through Craigslist for $150. Their copper tubing was worth more than $75 each, but they’d been sitting in a storage for over a decade. (I was even able to trace the panel’s label plates to the California company that made them in 1976.)
As a submariner I’ve done plenty of piping flushes, fills, and hydrostatic tests so we hauled them home, flushed them, and connected them to our garden hose. I was thrilled to see them hold pressure so we hauled them up to our second-story roof and mounted them on more angle brackets. Craigslist yielded a used solar water heater & pump for $150 and we spent another $600 on materials like copper pipe, valves, a controller, the connectors, and insulation. Our 13-year-old daughter helped us plan & lay out the design and I taught her how to solder copper pipe. (Today she’s finishing her senior year of a civil engineering degree.) Our system immediately started making more hot water than we ever use.
In 2006 we took more state/federal tax credits, and in 2007 we took our final credits. Between the state/federal tax credits and Oahu’s electric rates of 30-35 cents/KWHr, the PV array and the solar water heater paid for themselves by late 2010. Sweat equity led to less than a six-year payback.
We’re frequently asked about breakdowns, repairs, and maintenance. Our solar systems have survived many hours of high winds, rainstorms and even a house-rattling 5.5 earthquake. They were just tested two days ago by Tropical Storm “Flossie”. The panel mounts & racks actually help strengthen the roof against hurricanes. We’ve never had a breakdown or a service interruption. If HECO goes down then our inverter (and our array) goes down too, but when HECO comes back up then the inverter reboots right away. We don’t even bother to clean the panels. If I’m on the roof for another reason then I’ll look them over, but no maintenance has been necessary. We don’t carry insurance on any of our solar hardware– just the usual homeowner’s policy with a high hurricane deductible.
Technology has come a long way in the last few years. Worldwide subsidies (and overseas mass production) have crashed the wholesale price of photovoltaic panels to about $1/watt. The industry has also developed better roof mounts (with aluminum flashing in a choice of colors!), better rack mounts & clips, and cheaper grounding systems. Microinverters greatly simplify assembly and expansion. There are thousands of solar websites to guide you through the code requirements in your area, the permit paperwork, the tax credits and rebates, the designs & hardware, and even to share reviews of the installation companies.
The Next Step: DIY Electric Vehicle Recharging
My spouse and I are very happy with our used Priuses, and we’ll drive them into the ground. (That may take another 20 years.) We drive less than 7000 miles per year and we spend under $800/year for gas. Electric vehicles (and next-generation hybrids like the Chevy Volt) have controversial paybacks due to their higher prices. Their payback is a little faster with Hawaii’s gas prices of >$4/gallon and by finding a good used EV through Craigslist. However, the payback will be even faster when we add a few more photovoltaic panels to our array. We might have to buy a bigger string inverter or set up a separate batch of panels on microinverters, but the charging circuit and hardware are already standard. Maybe we’ll hire an electrician to help us wire them into the garage, but it might not be necessary.
Is this feasible? Yes, our neighbor has been recharging her Nissan Leaf from her house’s net-metered photovoltaic array and commuting up to 80 miles per day. Her PV array makes extra power during the day and dumps it into HECO’s grid, then draws power from the grid at night to recharge her Leaf’s batteries. Not only does she have a $17/month electric bill, but she drives nearly 8000 miles/year and never buys gasoline!
Are you considering your own photovoltaic array or solar water heating system? If DIY is not for you, then read this article from Scientific American on choosing an installation company and this article from Business Week about shopping for solar panels.
Extreme home improvement: the finances of your residential photovoltaic array
Guest Post Wednesday: The bright side of solar power purchase agreements
DIY home maintenance
DIY home improvement
Lessons learned from DIY home improvement
Scientific American: “How to shop for solar array installers”